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Trapping flow into pit channels in steep terrain

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Details of my model:

- 4m grid size

- Q type pit channels to take flow from the street network into the 1d drainage network

- H-Q database of accurate H-Q relationships, for side entry pits, grates, etc

- pit channels have an SXL flag & US_Invert level used to lower the cells ~0.1 below the surrounding terrain.

- SA points to input the flow hydrograph from each sub-catchment - these are applied to the same cell as each pit channel.

I'm finding that when the 2d slope is steep (i.e. allotments fall steeply away from road, sometimes with surrounding ZC points up to 3m lower!) flow tends to spill into downstream cells rather than through the pit channel into the 1d network. This is probably due to the flow spilling rather than building up a ponding depth on the cell for the H-Q curve to apply.

I'm trying to lower the pit channel cells below the surrounding cells to create some ponding depth. However lowing them by too much messes up the H-Q relationship, puts cells below the 1d_network, etc, etc. My question is do I need to lower the ZC points below the ZC levels of surrounding cells or just the ZU & ZV side points of the pit channel cell?

Some help would be appreciated - thanks!

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It sounds like the hydraulics are correct, ie flow is running down the hill too fast to enter the pits. The momentum will continue the flood water downhill rather than allowing it to enter the pit.

Considering reality, have you tried amending the 2D domain to provide a small horse-shoe shaped berm round the pit entry. This is probably what would be needed to "catch" the water in reality. This will lower momentum enough to allow entry into the pit

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Yes we took our pit relationships from some calibrated relationships we had in Drains - though it would be very nice if TUFLOW could automatically work out if the pit was on grade or sag and apply the appropriate relationship (I appreciate this would be difficult).

I have been using Z_lines to create little banks around the low sides of pits that weren't capturing flow (to 0.1m above the pit ZC level). The only thing is that out of the ~3100 pits in my model, there's over 100 that I have to manually do this for . . . a nice new feature would be a flag on the pit channel to do this automatically.


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